1. Field of the Invention
This invention relates in general to earth boring machines, and in particular to a cable tensioner for a downcrowding device for use with earth boring machines.
2. Description of the Prior Art
In U.S. Pat. No. 4,150,727, a device is shown for applying downcrowding force on a cutting tool. The device includes a drum with a brake, hydraulic rams to move the drum vertically, and a downcrowding cable extending directly from the drum to the kelly. To apply downward force to the kelly, the brake is set and the drum is moved downwardly with the hydraulic rams to pull downwardly on the downcrowding cable and kelly.
When moving the drum upwardly with respect to the kelly to start a new stroke, a take-up cable is used to take up slack in the downcrowding cable. The take-up cable is wrapped around the drum, reeved over a sheave in the boom, and is connected to a spring cable tensioner at the top of the kelly. When the kelly and drum move downwardly, the length of the cable tensioner increases. When the brake is released on the drum and the drum is moved upwardly for a new stroke, the cable tensioner retracts, rotating the drum and taking up slack. The take-up cable thus increases in length, and the downcrowding cable decreases in length.
During downcrowding, the cable tensioner's length is increased equal to the kelly displacement plus the drum's displacement. Consequently, the cable tensioner must be able to expand and retract a distance twice that of the kelly displacement. While this take-up system is satisfactory, reducing the expansion and retraction distance of the cable tensioner would be advantageous.